In numerous processes in the chemical industry, gas streams occur which contain acid gases, for example CO2, H2S, SO2, CS2, HCN, COS or mercaptans as impurities. These gas streams can be, for example, natural gas, synthesis gas from heavy oil or heavy residues, refinery gas or reaction gas formed in the partial oxidation of organic materials, for example coal or petroleum. Before these gases can be transported or further processed, the acid gas content of the gas must be markedly reduced. CO2 must be removed from natural gas, for example, since a high concentration of CO2 decreases the heating value of the gas. Furthermore, CO2, together with water frequently entrained in the gas streams can lead to corrosion on pipes and fittings.
The removal of sulfur compounds from these gas streams is of particular importance for various reasons. For example, the sulfur compound content of natural gas must be reduced by suitable processing measures directly at the natural gas source, since the sulfur compounds, in the water frequently entrained in the natural gas, also form acids which have a corrosive effect. To transport the natural gas in a pipeline, therefore, preset limiting values of sulfur contaminants must be maintained. Furthermore, numerous sulfur compounds are foul-smelling even at low concentrations, and, especially hydrogen sulfide, are toxic.
It is known to remove the unwanted acid gas constituents from the gases by gas scrubbing with aqueous or nonaqueous mixtures of organic solvents as adsorbents. In this process both physical and chemical solvents are used. Known physical solvents are, for example, cyclotetramethylenesulfone (sulfolane), N-methylpyrrolidone and N-alkylated piperidones. The chemical solvents which have proven themselves industrially are, in particular, the aqueous solutions of primary, secondary and tertiary aliphatic amines and alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), monomethylethanolamine (MMEA), diethylethanolamine (DEEA), triethanolamine (TEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA).
Tertiary alkanolamines do not react directly with CO2, since the amine is completely substituted. Rather, CO2 is reacted in a slow reaction with the tertiary alkanolamine and with water to form bicarbonate. Because of the slow reaction of the CO2, in the gas scrubbing, the scrubbing process must be carried out using tertiary alkanolamine solutions having a high liquid/gas ratio with a correspondingly high solvent recirculation. Therefore, attempts have been made to increase the absorption rate of carbon dioxide in aqueous solutions of tertiary alkanolamines by adding further compounds which are termed activators or promoters.
Promoters of this type can be primary and secondary amines. A preferred promoter is piperazine. The nitrogen of the primary and secondary amines reacts directly with carbon dioxide, forming carbamate. In the aqueous amine solution, the carbamate is in equilibrium with bicarbonate. To regenerate the amine solution, in which carbon dioxide and if appropriate other absorbed acid gases are removed by expansion and steam stripping, a large amount of steam and correspondingly a large amount of thermal energy are required to decompose the carbamate. Furthermore, the solubility of piperazine in aqueous solution is low.
Therefore primary and secondary amines, such as piperazine, have previously always been used in very small amounts, that is to say generally less than one mol/l of absorbent. For example, there are numerous absorbents which contain methyldiethanolamine and piperazine in combination in aqueous solution, the piperazine concentration being less than 1 mol/l of absorption solution (U.S. Pat. No. 4,336,233; DE 25 51 717; U.S. Pat. No. 4,551,158; U.S. Pat. No. 4,553,984; U.S. Pat. No. 4,537,753; U.S. Pat. No. 4,999,031; CA 1,291,321; EP 202 600; EP-A 159 495; EP-A-190 434).
Absorbents having higher contents of secondary amine, such as piperazine, are disclosed in the following publications:
EP-A-0 008 449 describes the purification of COS-containing gases or liquids by hydrolyzing the COS using aqueous solutions containing N-heterocyclic compounds. The N-heterocycle present can be up to 75% by weight piperazine, and in addition up to 50% by weight of methyldiethanolamine (MDEA).
WO 00/00271 discloses an absorbent which contains from 0.1 to 50% by weight of nitrogen heterocycles, for example piperazine, from 1 to 60% by weight of an alcohol, from 0 to 60% by weight of an aliphatic alkanolamine, for example methyldiethanolamine, from 0 to 98.9% by weight of water and from 0 to 35% by weight of K2CO3. The absorbent has a high absorption rate and high capacity for acid gas constituents. The absorbent is particularly suitable for gases to be purified which contain CO2 in amounts up to 75% by volume and H2S in amounts up to 50% by volume. Furthermore, the gases to be purified can contain COS, preferably in amounts up to 5% by volume, CS2, preferably in amounts up to 1% by volume, and mercaptans, preferably alkylmercaptans, in amounts up to 1% by volume.
EP-A-0 879 631 discloses a process for removing CO2 from gases, inter alia using aqueous solutions of a secondary amine, for example piperazine, and of a tertiary amine, for example methyldiethanolamine. Both amines can be used at concentrations in each case of from 10 to 45% by weight, in which case the total concentration is not to exceed 70% by weight. The gases purified in the examples contain 10 mol % CO2.
EP-A-0 359 991 discloses a process for removing CO2 and if appropriate H2S from gases, using aqueous absorption solutions containing tertiary alkanolamines, in particular methyldiethanolamine. Preferably, in addition, a secondary amine such as piperazine may be present. The tertiary amine is preferably present in the absorption solution in an amount of from 20 to 70% by weight, and the secondary amine in an amount from 0.05 to 3 mol/l (from 0.4 to 24.6% by weight). The gases to be purified are to have a CO2 content of from 1 to 90 mol %, particularly preferably from 3 to 60 mol % by weight. In addition to CO2, the gases to be purified can contain, as further acid gases, H2S, for example in amounts of from 1 mol ppm to 50 mol %.
WO 00/66249 discloses an absorbent and a process for removing acid gases from gas streams which can contain, for example, CO2, H2S, COS, SO2, SO3, CS2, HCN, O2 and mercaptans. The absorbent is considered suitable for gases which are contaminated by from about 10 to 10,000 ppmv of mercaptans, up to 90 mol % H2S, up to 50 mol % CO2, from about 2 to 10,000 ppmv COS. The absorbent contains more than 1 mol % piperazine, preferably more than 1.1 mol %, particularly preferably more than 1.2 mol % piperazine per liter of aqueous absorbent, and from 1.5 to 6 mol % of methyldiethanolamine (MDEA) per liter of aqueous absorbent. Toward MDEA- and piperazine-containing absorbants which contain less than 1 mol/l of piperazine, the absorbent is said to ensure, at a decreased absorbent recirculation rate and higher absorber temperature, appropriate CO2 removal from the contaminated gas and improved behavior during the downstream regeneration.
The known absorbents which can contain piperazine at a concentration of at least 1 mol/l of absorbent are intended for treating gases which have high concentrations of impurities due to acid gases. For example, the acid gases in natural gas purification are generally present at a partial pressure in the range from 3 to 7 bar (at a total pressure of from about 50 to 70 bar). However, frequently, gases must also be purified which have only low levels of acid gas impurities, that is to say the partial pressure of the acid gases in the gas or gas mixture to be treated is low. In the case of acid gases present at only low concentrations, during the extraction under customary conditions, only a low equilibrium loading of the absorbent is achieved. To remove the acid gases, therefore, the use of relatively large amounts of absorbents is necessary.